This invention relates to a method for controlling agglomeration of a caking coal so that the coal can be pyrolyzed in a continuous process without plugging the pyrolysis reactor.
The use of fluidized systems wherein a fluidized stream is formed of finely divided coal particles, heated char particles and a carrier stream to pyrolyze the coal particles to extract the volatiles therefrom is known in the art. In such prior art processes the heated char particles provide at least a portion of the requisite heat of pyrolysis to the coal particles with a supply of char continuously being produced upon pyrolysis of the coal in the system.
Agglomerative particulate bituminous coals are known to those skilled in the art to plasticize and become sticky or tacky at low temperatures, e.g., 400.degree. to 850.degree. F. Application of such prior art processes to agglomerative bituminous coal results in problems due to the agglomerative nature of such coal. When agglomerative coal particles are heated to their plastic state, and the heated particles contact a wall of a reactor, the particles can cake thereon to form a bubbly, compact mass which swells and then resolidifies, forming a solid coherent body with a porous structure, i.e. coke. Such agglomeration of coal particles on the reactor walls causes severe blockage in the system and renders the system inoperable.
To overcome plugging problems encountered in pyrolysis systems utilizing agglomerative coals, various procedures have been suggested by the prior art. In U.S. Pat. Nos. 2,955,077 and 3,375,175 an agglomerative particulate coal is preheated in a fluidized bed at temperatures ranging from 600.degree. F. to 825.degree. F. for from 1 to 30 minutes to remove at least a portion of the volatiles from the coal so the coal can be further pyrolyzed to recover the volatiles therefrom. The requirement of preheating agglomerative bituminous coals for long residence times imposes severe economic limitations on these processes.
In U.S. Pat. No. 3,736,233 there is disclosed a continuous process for pyrolysis of agglomerative bituminous coal by heating the particulate coal having a particle size of less than 65 microns with char, both of which are entrained in a carrier gas having pyrolysis reactor residence times of under three seconds. This patent also suggests that it may be helpful to use a reactor having porous walls through which a gas is continuously passed to prevent sticking of particles to the reactor walls.
U.S. Pat. No. 3,357,896 discloses heating large particles of caking coal through their plastic range in a free fall system to avoid contact with the reactor walls and to produce noncaking coal char. The patent also discloses the use of oxygen in the heating gas to prevent caking of the coal while it is heated through its plastic range. Such treatment with oxygen has the disadvantage that it substantially reduces the yield of hydrocarbons produced during pyrolysis.
Still another prior art process employs sodium carbonate to decrease agglomeration of the coal.
Other processes for making noncaking coal and chars from highly caking coals are complex and require expensive mechanical devices such as rotating kilns, chain grates, jiggling grates, and rotating screws to prevent caking coals from fusing into one solid mass while being taken through the plastic temperature range. Such equipment makes these conversion processes expensive.
To applicant's knowledge, none of the prior art processes has proved completely satisfactory, and very few of such prior art processes are practiced commercially.
Therefore, there is need for an efficient, economical, continuous method for pyrolyzing agglomerative coals in a transport reactor for recovery of volatile hydrocarbons under conditions which prevent plugging of the reactor.